Let me try and answer a few of them. But before that, can you let us know which service you are planning to use? The reason I ask this is there are some of the services which are very much real time (like Email verification). i.e. the requests are checked/verified against the actual email domains and not a cached lookup against an internal database. This would be the case with most of the services as they incur a real-time entity.
Having said this, answers below.
1 - There are no specific limits on the number of requests being sent to SI services, if that is what you are referring to.
2 - We do not have a specific defined throughput number- the primary reason being the real time nature of the service calls.
3 - generally within a second. but most of the times the responses are very quick in the matter few milliseconds. It would be like a click and a response
4 - Not sure about this question. Since there are multiple parties involved in the request-response sequence, the any of them could cause a performance issue. if it is something that SI as a product is doing, we would definitely be involved to take it further.
Hope this helps.